It is often necessary to monitor the relative humidity of a gas. For example, product manufacturing processes often include one or more steps which must be carried out within a tightly controlled humidity range. Also, in order to maximize the shelf life of some products, it is necessary to minimize humidity in the atmosphere where the product is packaged. Another example where it is important to monitor relative humidity levels is in greenhouses. Additionally, it is important to monitor relative humidity levels in an incubator which contains a subject, such as a prematurely born infant, where temperature, humidity and oxygen levels must be carefully controlled. In such a system, it is often desirable to monitor the subject's rate of metabolism which is relatable to the subject's evaporative heat loss, which in turn affects the humidity level of the gas in the incubator.
Numerous devices have been used in the past to monitor the humidity of a gas. One class of humidity measuring devices includes psychrometers. U.S. Pat. No. 4,461,167 describes numerous examples of psychrometer devices. Psychrometers generally employ two thermocouples, one of which monitors the dry-bulb temperature of a gas flow, and the second of which is embedded in a moistened wick along the same gas flow. Changes in humidity should not affect the dry-bulb temperature of the first thermocouple. However, the temperature reading of the second thermocouple is affected by changes in humidity of the gas because, depending on the relative humidity of the gas, more or less liquid evaporates from the wick. Therefore, the difference between temperature readings of the first and second thermocouples indicates relative changes in humidity of the gas.
There are several significant disadvantages which present themselves with psychrometers of the prior art. First, the "second" thermocouple's sensing of evaporative heat loss from the moistened wick is relatively indirect because evaporation primarily occurs at the wick's surface, whereas the thermocouple's sensor resides in a sub-surface, internal portion of the wick. Temperature changes inside the wick which are due to evaporative heat loss are diminished in magnitude and delayed in comparison to temperature changes which occur at the wick's surface. Another disadvantage with psychrometers in the prior art is that their response to a change in humidity of a gas is relatively slow, i.e., the time it takes for a temperature change at the wick's surface to cause a temperature change inside the wick. The heat transfer process may take up to several minutes depending on how deeply into the wick the thermocouple sensor penetrates. A psychrometer which takes minutes to detect humidity changes cannot be used effectively in situations where humidity levels are changing rapidly, i.e., fractions of minutes or seconds.
Another type of device which has been used to measure atmospheric humidity is referred to as a hygrometer. In a hygrometer, for example, the one disclosed in U.S. Pat. No. 3,636,768, identical infrared beams pass through two enclosures, one of which contains dry air, and the other of which contains sample air whose humidity is to be measured. An infrared detector receives the two beams. The difference between the levels of detectable infrared light from the two beams indicates the relative humidity level of the sample gas because water molecules in the sample gas absorb infrared light. A hygrometer is relatively more complex than a psychrometer because it requires a light source and a dry gas source. The relative dryness of the dry gas source must be tightly controlled and monitored, otherwise the humidity determination of the sample gas may be invalid or unreliable.
Accordingly, one object of the present invention is to provide a device capable of measuring relative humidity in a gas, rapidly, and preferably instantaneously.
Another object of the invention is for the device to measure relative humidity without requiring a source of dry gas for comparison.